Driver for non-linear displays comprising a random access memory for static content

ABSTRACT

Driver system ( 10 ) for use in connection with a non-linear display array having N×M pixels. The driver system ( 10 ) comprises an input for receiving column data representing an image to be displayed, and a row driver designed to sequentially collect the currents of all M pixels of each row of pixels row electrode by row electrode. A gamma correction unit ( 17 ) is employed that provides for a gamma correction of the column data. The gamma correction unit ( 17 ) is situated at the input side of a display data memory ( 14 ) for storing the gamma corrected column data. The driver system ( 10 ) further comprises a column driver ( 24 ) designed to apply column signals to all M column electrodes in parallel, the column signals being generated in accordance with the gamma corrected column data.

The present invention relates to non-linear display systems, such aspolymer light emitting displays that require a random access memory forstoring static image content, and in particular drivers for displays ofthis kind.

With the widely divergent use of electronic devices comprising displays,for instance laptop computers and mobile phones, various displaytechnologies have been employed, for example liquid crystal displays(LCD), light emitting diode (LED) displays, and more recently organiclight emitting (OLED) displays.

Cathode ray tubes (CRT) and thin film transistor (TFT) matrix displaysare further examples of display technologies widely used. Since CRT andTFT displays have a non-linear characteristic, a gamma correction isperformed in order to adjust the image being displayed accordingly.These displays are mostly employed in devices where the display contentchanges dynamically and there is thus no need for a display data memory.

The OLED technology holds promise because of its ability to efficientlyaddress a very wide range of colors, while operating at extremely lowpower. As a result, this technology is expected to be brighter, lower incost, consume less power (which is an advantage if used in portableelectronic devices which depend on a battery as a power source), affordwider viewing angles, and be extremely lightweight. OLEDs are thus idealfor today's mobile device applications. Moreover, this technology willbe also ideal for a variety of lighting conditions and capable ofrunning at fill speeds in extreme temperatures.

Polymer light emitting diodes (PolyLED), a segment of the total OLEDmarket, will be a key display technology in the future, especially incolor mobile applications.

Some of the essential parts of a conventional matrix driver 1 areillustrated in FIG. 1. The driver 1 is a single chip driver that can beused for driving a passive-matrix PolyLED display featuring N=64 rowsand M=102 columns, i.e. 64×102 pixels. The driver 1 comprises columndriver means 2 and row driver means 3. The current to be supplied to thelight emitting diodes of the PolyLED display is furnished by a DAC 6(digital-to-analog converter) that converts a number received from aninterface into an appropriate intensity of a current Icol. This currentIcol will be mirrored via the column driver 2 to the columns of thePolyLED display. The row driver means 3 collect the currents of theanodes of the light emitting diodes of a whole row. The column drivermeans 2 are current sources. Means 4 for gamma correction are providedat an output side of the display data memory 5. Different grey levelscan be obtained using the PWM unit 7.

The power consumption of current display drivers for use in connectionwith non-linear displays is still an issue, and there is a demand fordisplay devices consuming even less power than conventional ones.

It is an object of the present invention to provide an improved driverfor a non-linear display and to provide an improved non-linear displaydevice.

It is an object of the present invention to provide a driver fornon-linear display, e.g. an electroluminescent display, that consumesless power than a conventional driver.

These and other objects are achieved by the present invention, whichprovides a driver system for use in connection with a non-linear displayarray. The driver system comprises a row driver designed to sequentiallycollect row-by-row the currents of the pixels of a row. This is done byapplying a row select signal to the N row electrodes to scan one rowafter the other. A gamma correction unit is employed for performing agamma correction of column data representing an image to be displayed.The gamma corrected column data are stored in a display data memory. Thedriver system comprises a column driver designed to apply column signalsto the M column electrodes in parallel, the column signals beinggenerated in accordance with the gamma corrected column data.

Also provided is a non-linear display (for example a passive matrix N×Mpolymer light emitting diode array) comprising a driver system.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

For a more complete description of the present invention and for furtherobjects and advantages thereof, reference is made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic block diagram of a conventional matrix driver thatcan be used for driving a PolyLED display;

FIG. 2 is a schematic block diagram of a matrix driver, according to thepresent invention that can be used for driving a PolyLED display;

FIG. 3 is a schematic block diagram of a PolyLED display;

FIG. 4 is a graph showing a brightness versus current curve of a PolyLEDdisplay;

FIG. 5 is a graph showing a brightness versus current curve of a PolyLEDdisplay and a segmentation of a row slot according to the presentinvention;

FIG. 6 is a schematic diagram showing a gamma correction unit, accordingto the present invention.

The present invention is described in connection with severalembodiments. In the following sections mainly polymer OLED (PolyLED)color display are being addressed. The invention, however, is alsoapplicable to any other type of non-linear display array.

A driver 10 in accordance with the present invention is illustrated inFIG. 2. The driver 10 can be used for driving a passive-matrix polymerOLED (PolyLED) color display that features N=64 rows and M=128×3columns, i.e. 64×128×3 pixels (note that three green, red, bluesub-pixels form one pixel). Such an OLED display may comprise a seriesof emissive polymer-based thin films sandwiched between two electrodes,one of which is transparent (most often glass). The thin films define amatrix of N×M light emitting diodes, each light emitting diode having ananode and a cathode.

An example of a monochrome PolyLED display 40 is depicted in FIG. 3.Only a few pixels are shown in FIG. 3 for the sake simplicity. Inpractice, there may be several hundred rows and columns of pixels. Thedisplay 40 comprises N=4 rows 42.1 through 42.4 and M=6 columns 41.1through 41.6. There are N×M light emitting diodes arranged in a matrixwith N=4 rows and M=6 columns, so that the anodes of all the lightemitting diodes of the first row connect to the respective row electrode42.1, all the light emitting diodes of the second row connect to therespective row electrode 42.2, and so forth. The cathodes of all lightemitting diodes of the first column connect to the respective columnelectrode 41.1, the cathodes of all light emitting diodes of the secondcolumn connect to the respective column electrode 41.2, and so forth. Inoperation, each row of light emitting diodes is sequentially activatedvia the corresponding row electrode 42.1-42.4, where the individuallight emitting diodes are activated using the corresponding columnelectrodes 41.1-41.6. A light emitting diode emits light if its cathodeis at 3.3 volts, for example, and its anode at the same time is at 0volt, since the diodes are reverse biased. In other words, as long as apositive voltage is applied to a row electrode 42.1-42.4, none of thediodes connected to this particular row electrode can be activated, nomatter what column signals are being applied to the column electrodes41.1-41.6. On the left hand side of FIG. 3, the timing of the row selectsignal r(t) and the column signals c1(t)-c6(t) is depicted. For the sakeof simplicity, only two column signals c2(t) and c4(t) actually showpulses. In the given example all other column signals c1(t), c3(t),c5(t), and c6(t) are at zero volt. The column signal c1(t) is applied tothe column electrode 41.1, the column signal c2(t) is applied to thecolumn electrode 41.2, and so forth, as illustrated in FIG. 3. During afirst time slot a, the row select signal r(t) is pulled to zero whilebeing applied to the first row electrode 42.1. Since during this timeslot a none of the column signals c1(t)-c6(t) is at 3.3 volts, all lightemitting diodes of the first row remain dark. During the time slot b therow select signal r(t) is at 0 volt while being applied to the secondrow electrode 42.2. At the same time the column signal c2(t) is at 3.3volts. This constellation of signals causes a current to flow throughthe light emitting diode 9.1 in row two and this diode 9.1 emits light.No other diode of the same row 42.2 emits light since only the signalc2(t) is at 3.3 volts in the given example. During a third time slot c,the row select signal r(t) is pulled to zero while being applied to thethird row electrode 42.3. Since during this time slot c none of thecolumn signals c1(t)-c6(t) is at 3.3 volts, all light emitting diodes ofthe third row remain dark. During the time slot d the row select signalr(t) is at 0 volt while being applied to the fourth row electrode 42.4.At the same time the column signals c2(t) and c4(t) are at 3.3 volts.This constellation of signals causes currents to flow through the lightemitting diode 9.2 in row two and the light emitting diode 9.3 in rowfour. The two diodes 9.2 and 9.3 emit light. Since the scanning of allrows 42.1 through 42.4 is done quickly, the human eye perceives allthree diodes 9.1, 9.2, and 9.3 to be turned on at the same time whileall other diodes remain dark. All three diodes 9.1, 9.2, and 9.3 shineduring one whole slot length w, i.e. all three diodes 9.1, 9.2, and 9.3emit at the maximum brightness.

The driver 10, as illustrated in FIG. 2, is designed to drive the rowelectrodes 42.1-42.4 and column electrodes 41.1-41.6 accordingly. Thecolumn data representing an image to be displayed on a display 40 is fedfrom a host, for example via a data link 11 and a buffer interface 12 tothe driver 10. The buffer interface 12 transforms the serial column datainto parallel column data. An address counter 13 is employed in order tobe able to write the column data byte-by-byte into a display data memory14. A random access memory (RAM) is used as display data memory 14. TheRAM 14 has a capacity of 64×128×16 bits, since in the present examplethe column data are coded on 16 bits (6 bits green, 5 bits red, 5 bitsblue).

In the present example, the buses 15 and 16 are 16 bits wide. Accordingto the present invention, a gamma correction unit 17 is employed. Thisunit 17 is situated in front of the display data memory 14 and isdesigned to transform the column data received via bus 15 into columndata that take into consideration the non-linear behavior of the lightemitting diodes of the PolyLED display 40. Such a gamma correction isnecessary since the relationship between the current fed through a diodeand the brightness of the light emitted by the diode is non-linear. Anexemplary current versus brightness curve is given in FIG. 4. This curveillustrates the non-linearity of the display 40. According to thepresent invention, the column data stored in the RAM 14 are correctedfor each color (green, red, blue). Data that have been processed by thegamma correction unit 17 are herein referred to as gamma correctedcolumn data.

These gamma corrected data are then fed via the bus 16 into the memory14. Optionally, data latches 18 are employed at the output of the memory14 to keep the gamma corrected column data for a short period of time.The gamma corrected column data are forwarded in several steps via thebuses 19, 20, and 21 and the units 18 (optional) and 23 to a columndriver 24. The buses 19 to 22 are 128×3 bit wide. A pulse control unit23 is situated at the output side of the RAM 14. This unit 23 transformsthe data representing the three colors green, red and blue intocorresponding grey levels. This may be done by controlling the length wof the column signals c1(t)-c6(t), for example. By doing so, the time arow is selected (active) and is divided into small slots which may be afraction of the slot lengths depicted in FIG. 3 as a, b, c, d, and e.These small slots may be of equal or unequal lengths.

There is a column driver 24 comprising switches (for example MOStransistors or bipolar transistors). These switches are employed toswitch a current Icol received from a current source 25. It is possibleto calibrate the current Icol via the input 26. Other than an LCDdisplay which is driven with voltage levels, the PolyLED display 40 isdriven with constant currents. An example of a switch suitable for usein connection with the present invention is described in the PCT-patentapplication WO 99/65012, as published on 16 Dec. 1999. This PCT-patentapplication is currently assigned to the assignee of the presentapplication.

A converter block 27 in provided for up- or down-conversion of thesupply voltage Vdd2 to the voltage Vh needed by the display 40 (in thepresent example, Vh=3.3 volts). The supply voltage Vdd2 may be providedby a battery. An oscillator 28, for example an RC-oscillator, providesthe timing signal needed by a timing controller 29. The timingcontroller 29 synchronizes the column signals c1(t)-c6(t) and the rowselect signal r(t). For this purpose, the timing controller 29 isconnected via links 30 and 31 to the RAM 14 and the row driver 32,respectively. The row driver 32 comprises switches (for example MOStransistors or bipolar transistors) that connect to the row electrodesof the display 40.

It is an advantage of the present invention that gamma correction ofincoming column data is only needed when the image on the display 40changes.

According to the present invention, a gamma correction unit 17 is, forexample, a logic block implementing a non-linear function. Its inputreceived via the bus 15 is a number represented with N bits (for exampleN=16) for a pixel color content. The output at the bus 16 is a numberrepresented on M bits (for example M=18) where M may or may not be equalto N. The output number M sets the exact length of the current pulse,created by the pulse control unit 23. The pulse control unit 23 is fixed(wired-up), whereas the gamma correction unit 17, according to thepresent invention, allows a certain degree of programmability, either toadapt to a specific electroluminescent material or to cope with thematerial's efficiency degradation in time.

In detail, the gamma correction unit 17 may be, in a preferredembodiment, a look-up table, but not exclusively: any digital processingunit with an N-bit number as input and an M-bit number as output andmimicking the non-linear characteristic of the display (cf. FIG. 4)might be used. An example of an embodiment of a gamma correction unit 60is illustrated in FIG. 6. The gamma correction unit 60 comprises aninput buffer 61 and an output buffer 62. The input buffer 61 receivesvia the bus 15 a number represented with N bits (raw column data). Aftergamma correction, the output buffer 62 provides gamma corrected data atthe output bus 16, the data being represented on M bits. The curveinside the box 60 in FIG. 6 represents the non-linearity that is to becorrected by means of the gamma correction unit 60.

In a preferred embodiment of the present invention, the gamma correctionunit 17 comprises a look-up table. The column data can be convertedwhile being written into the RAM 14. The advantage is that this look-uptable is addressed while the column data are being transferred orchanged only if a change is necessary. Otherwise the content of the RAM14 remains static. As long as the image on the display 40 remainsstatic, no gamma correction needs to be performed. The gamma correctionis only carried out when new column data are received via the input 1,and not during each time slot, as in a conventional driver illustratedin FIG. 1.

The inventive approach saves computing logic, time and power, since eachgamma correction would consume power.

Another embodiment is characterized in that a timing generator isemployed that generates unevenly distributed time slots w(t). An exampleof a current vs. brightness curve 50 is depicted in FIG. 5. Below thiscurve 50, the distribution of the time slots is plotted in a time vs.current diagram. The various lengths w(t) of these time slots have to betaken into consideration when performing the gamma correction. In thiscase, a connection between the timing generator and the gamma correctionunit is required. The distribution of the slots w(t) is chosen such thatfor the steep part of the curve 50 there are many short slots w(t),whereas for the flat part of the curve there are fewer but longer slotsw(t). A column signal c(t) is shown in FIG. 5. This signal c(t) is sevenslots wide in the given example. The width of this signal c(t) resultsin a brightness b1. A signal c(t) whose width is equal to the length ofa row slot would result in the maximum available brightness.

In another embodiment, a separate unit is provided before the displaydata memory in order to provide for a compensation of the degradation ofthe light emitting diodes. There is a correlation between the currentflowing through the individual diodes and their efficiency. Ananalytical expression of the relationship of the current vs. lightoutput may be used to determine an additional charge to be injected intoa particular LED for it to maintain a (substantially) constant lightoutput. A look-up table, preferably a table sampled in time, may becomprised in the separate unit in order to account for this degradationbefore storing the column data in the display data memory.

The driver according to the present invention offers an integrated DC-DCconverter and oscillator, multiple serial and parallel high-speed businterfaces, and an integrated gamma correction solution that is fast andefficient.

Drivers in accordance with the present invention can be used insmall-scale mobile applications including cellular phones, pagers,digital cameras, PDAs, and so forth.

It is appreciated that various features of the invention, which are, forclarity, described in the context of separate embodiments may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment may also be provided separately or in anysuitable subcombination.

In the drawings and specification there have been set forth preferredembodiments of the invention and, although specific terms are used, thedescription thus given uses terminology in a generic and descriptivesense only and not for purposes of limitation.

1. Driver system for use in connection with a non-linear display arrayhaving N×M pixels, the driver system comprising an input for receivingcolumn data representing an image to be displayed, a row driver designedto sequentially collect the currents of all M pixels of each row ofpixels row electrode by row electrode, by applying a row select signalto the N row electrodes so as to scan the rows of pixels one after theother, a gamma correction unit providing for a gamma correction of thecolumn data, a display data memory for storing gamma corrected columndata, a column driver designed to apply column signals to all M columnelectrodes in parallel, the column signals being generated in accordancewith the gamma corrected column data.
 2. Driver system of claim 1,wherein the non-linear display array is a polymer light emitting displaycomprising N row electrodes, M column electrodes, and N×M light emittingdiodes, each light emitting diode having an anode and a cathode, thelight emitting diodes being arranged in N rows and M columns.
 3. Driversystem of claim 1, wherein various grey levels are obtained by employingrow select signals and/or column signals having pulses of differentpulse length.
 4. Driver system according to claim 1, wherein the gammacorrection unit comprises a look-up table.
 5. Driver system according toclaim 4, wherein entries in the look-up table take into considerationthe non-linearity of the non-linear display arrays.
 6. Driver systemaccording to claim 4 in combination with claim 2, wherein entries in thelook-up table take into consideration the non-linear brightness versuscurrent characteristics of the light emitting diodes and the sensitivityof the human eye.
 7. Driver system according to claim 3, wherein thelight emitting diodes are arranged so that the anodes of M out of theN×M light emitting diodes connect to one of the N row electrodes whereaseach of the cathodes of said M out of the N×M light emitting diodesconnects to a different one of the M column electrodes.
 8. Driver systemaccording to one of the preceding claims, further comprising a pulsecontrol unit which renders it possible to display different grey levelson the display.
 9. Driver system according to one of the claim 1,wherein the gamma correction unit comprises a logic block implementing anon-linear function.
 10. Nonlinear display array comprising a driversystem according to one of the claim
 1. 11. Polymer light emitting diodearray comprising a driver system according to one of the claim 1.